Insulated electrode construction for spark plugs



Dec. 17, 1935. H. RABEZZANA INSULATED ELECTRODE CONSTRUCTION FOR SPARK PLUGS Filed July 18, 1934 Patented Dec. 17, 1935 UNITED STATES msUtA'rEn ELECTRODE CONSTRUCTION FOR SPARK PLUGS Hector Rabezzana, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application July 18, 1934, Serial N0..735, 725

3 Claims.

My invention relates to spark plugs for internal combustion engines,'and particularly to center wire or insulated electrode features of such devices which electrode is supported by 5 and extends through a passage in the insulator of the plug. I

It is of extreme importance in spark plugs that expansion of the insulated electrode shall not crack or otherwise injure the insulator, to

which end the inner part of the insulated electrode extends loosely through a passage provided for it, but is not cemented within said passage; so that said inner part is free to expand and contract without subjecting the insulator to strain due to difierences in expansion between the metallic electrode and .the insulator, which is commonly made from porcelain or equivalent ceramic material.

Porcelain and equivalent material insulators are provided with a longitudinally extending passage for the center wire which has to be formed before the insulators are subjected to the usual-firing operation; and they are often distorted during the firing so that the passage,

though initially straight, is often more or less crooked after firing. A further object of my invention therefore contemplates an insulated electrode which may be placed within a more or less' crooked passage without subjecting the insulatorto strain'and perhaps cracking it; and

to this end the center wire or insulated electrode is made in two parts so that each may'adapt it-' self to the portion of the passage through which it extends. The inner part of this insulated electrode passes loosely through the inner part of the passage and need be but slightly smaller in diameter than the passage, whereas the outer part thereof is cemented in place within the outer part of the passage; the lower end of the outer part of the electrode abutting against, but

not being mechanically connected with, the upper end of the inner part of the electrode. The lengths of the two parts which form the center wire or insulated electrode are such that the lower end of the upper part abuts against the upper end of the lower part at a point within the enlarged central part of the insulator, which .central part is held within the external 50 metallic shell or casing of the plug and whereby a further feature of advantage in spark plugs is secured; the same being a more efiective dissipation of heat from'the upper end of the lower electrode part to a massive central part of the 55 insulator, and from said part and by as short and direct a path as is possible to the external shell or casing of the plug.

The above and other features of advantage in spark plugs are secured by the structural features illustrated in the accompanying drawing, 5 although my invention may be. embodied in other forms than the particular one illustrated; and I therefore vregard my invention as including such variations and modifications of the particular form illustrated as come within the scope 1 of the concluding claims wherein the features wherein my invention consists are particularly pointed out.

My invention as disclosed and claimed in this present application is in the nature of an im- 15 provement in one of the forms of insulated electrode or-center wire construction disclosed and claimed in my application Serial Number 676,996 for a Spark plug, filed on June 22, 1933.

In the drawing: 20

Figure 1 is a view showing a section upon a central longitudinally extending plane, .of a spark plug having center wire or insulated electrode features in accordance with my invention.

Figure 2 is a perspective view showing a fin- 25 ished flat metallic blank before it is bent into tubular form to provide a tubular terminal and holding member. I

Figure 3 is a perspective view of a tubular terminal and holding member formed by bend- 30 ing the blank illustrated in Figure 2 into tubular form.

eferring now to the drawing for a more complete understanding of my invention, the

numeral 4 designates the external metallic shell 35 or casing of a spark plug; and 5 the insulating member thereof-made of porcelain or equivalent material and which issecured within the shell in any way, as by an intumed flange 6 of the 4 shell. This insulating member has an enlarged 40 central portion as shown, and suitable gaskets '1, 8 are provided at the upper and lower ends of the enlarged part asis usual in spark plugs.

The insulating member is provided with an internal ledge 9 within its enlarged central part; with a lower electrode passage l0 leading from said ledge to the inner end of the insulator; and with a larger terminal and holding ember passage ll leading from said ledge to the outer end of the insulating member.

The numeral l2 designates a tubular terminal and holding member. located within the passage H and extending beyond the upper end thereof, as shown, said upper end being threaded at I3 to receive a nut for securing a supply conductor 55 thereto; and the lower end of this member is provided with means whereby it may be secured in place within said passage ll. As shown said terminal and holding member is provided with lugs l4 adjacent its inner end which interlock with an internal thread within the passage II, but any suitable means may be used for securing said member in place by an interlocking action between it and the inner wall of the passage through which it extends.

The numeral l5 designates the insulated metallic electrode proper of the plug as the member [2, while it forms a part of the center wire assembly whereby current is conducted to the spark gap of the plug, is herein regarded primarily as a means for holding the electrode IS in place within the insulator and for conducting electricity to said electrode. This electrode has an-enlarged head l6 which rests upon the ledge Sand against. which it is held by the terminal and holding member I2, the lower end of which abuts against said head and forms a metallic contact therewith; and the same extends through the passage in and beyond the lower end of the insulator and into sparking relation with the grounded electrode I! of the plug. The electrode I5 fits loosely within the passage I!) but said passage is larger than the electrode only to an extent sufficient to permit the same to be conveniently assembled with the insulator and it is contemplated that there shall be no cement between it and the inner wall of the passage; from which it follows that the electrode is free to expand independently of the insulator as it becomes heated, and that expansion and contraction thereof will not subject the inner end of the insulator (which expands very much less than the electrode) to strain as would be the case if the electrode was cemented in placewithin the passage through which it extends.

The tubular terminal and holding member l2 may be made from suitable bar or tubular stock, and by the use of any machines or devices suitable for imparting to it the form and features hereinbefore enumerated; although as illustrated it is preferably formed from a fiat sheet metal blank by bending the same into tubular form. Such a blank is shown in Figure 2 after a series of parallel corrugations l8 have been formed at one end thereof, and a serieslof projections M at the other end formed by displacing portions of the metal of the blank; after which and when the blank has been bent into tubular form its sides abut with one another along the line 20, Figure 3. When finished by bending as aforesaid the corrugations I8 form the threaded end l3 of the member l2, and the projections l9 provide the holding lugs i4 whereby said member is held within the passage II by engagement of said lugs with the thread within said passage. The end of the blank adjacent the projections I9 is formed with a projection 2| which when bent at right angles forms a closure 22 for the lower end of the tubular terminal and holding member l2, and provides a better and more extensive contact between the lower end of said 'member and the head l6 of the electrode l5.

Said blank is also formed with notches 23 which provide holes through which cement supplied to the interior of said holding member under pressure may flow outward and upward along said member, thus filling the interstices between the lugs and the thread and securing a gas-tight joint at the lower end of said tubular terminal and holding member. Ordinarily a suflicient amount of cement 24 is supplied to fill both the interior of the member l2 and the space between it and the passage II, as shown in Figure l of the drawing; and it will be understood that the cement is supplied as aforesaid after the member l2 has been assembled within the passage H and locked in place by the lugs I4, with its lower end in engagement with the head l6 of the electrode l5 to thereby hold said electrode in place within the lower passage I0. In the making of spark plugs the porcelain or equivalent insulating members are made into substantially theirfinal form, and the passages 10, II and the ledge 9 are provided, before the members are glazed and fired to vitrify them. The material being quite soft it follows that the passages are often not perfectly formed, and may not always be parallel and in alignment with one another; and these and other imperfections are liable to be increased during the firing necessary to vitrify the insulators. If then the center wire of the plug is a single unitary structure so formed initially, or formed by welding theparts thereof together, the same is likely to be more or less crooked or otherwise out of shape as when 25- the axes of the parts do not coincide. Thus and unless the insulating member and center wire assembly approach perfection the assembling of the parts is more or less diflicult, and is often accompanied by bending of the center wire and the :0 setting up of strains which tend to break the insulator; this breaking of the insulator or the setting up of strains tending to break it often occurring after the plug is finished as the strains may not be followed by immediate failure of the insulating member. A quite common way of attempting to avoid strain in the insulating member has been to make the passage for the lower end of the single piece center wire or insulated electrode structure much larger than the part of the structure which lies therein; but this necessitates the filling of the passage around said part with cement thus introducing a condition wherein longitudinal expansion of said lower part, due to the heating thereof when the plug is in use, is likely to break the lower end of the insulating member; because, as will be appreciated, the material of the insulating member has a negligible coeflicient of expansion and expands little if at all, whereas the lower end of the insulated electrode structure, being of metal, expands appreciably when the plug is in use and is heated by the burning gases within the combustion chamber of the engine.

In my invention and because the inner part [5 of the insulated electrode structure (made up of the parts It and I5) is independent of the outer part l2 the two may be assembled in'passages which may be considerably out of alignment or otherwise lacking in perfection, without straining either part or setting up stresses in the insulating member. Furthermore and as will be appreciated, the passage forthe inner part or electrode proper l5. may be made of less diameter than heretofore as it need be larger than said part only to such an extent as to permit the same to be put in place, thus making it unnecessary to surround such inner part with cement and avoiding injury due to unequal ex- 0 pansion of the insulating member and said part when the plug is in use.

Furthermore and because a comparatively short length only of the lower end of the tubular terminal and holding member I2 is rigidly interlocked with the insulating member, the liability to injury at this part of the insulator by expansion of the short length thus interlocked, due to the heating thereof by conduction from the head l6 of the inner electrode part I5, is greatly reduced. As a matter of course the lower interlocked part of the member l2 does not attain anything like the temperature of the electrode l5 because the greater part of the heat which might otherwise be communicated to it is dissipated to the massive central part of the insulating member, and to the shell of the plug and to the engine structure; but nevertheless a decided advantage as regards the avoidance of strains in the insulating member is secured by making this interlocked part short as compared with the total length of the tubular terminal and holding member.

The fact that the head l6 of the inner electrode part I5 lies within the enlarged central part of the insulating member, as does also the lower end of the tubular member [2 which is in direct and intimate contact with said head, results in a rapid dissipation of heat from said parts to the massive central part of the insulating member and therefrom to the metallic shell or casing of the plug and to the atmosphere and engine structure, thus securing an effective conduction. of

heat away from the electrode part [5 and the operation of the plug at a lower temperature than would be the case were such features of structure and arrangement of parts not present.

Having thus described and explained my invention I claim and desire to secure by Letters Patent:

1. A terminal and holding member for spark plugs comprising a fiat metallic blank bent into tubular form; said member having a threaded portion at one end, and securing lugs at the other end thereof. 10

2. A flat metal blank for use in forming a tubular terminal and holding member for spark plugs; said blank having a-plurality of parallel corrugations at one end, and a plurality of projections at its other end; the end of said blank 15 adjacent said projections being so shaped as to provide a closure for the open end of a terminal and holding member formed by bending said blank into tubular form.

3. A fiat metal blank for use in forming a tu- 20 bular terminal and holding member for spark plugs; said blank having a plurality of parallel 

